Biology of B and T lymphocytes: origin and surface molecules

B and T. Lymphocytes

The Biology of B. And T. Lymphocytes and the Reactions between Them

The Biology of B. Lymphocytes

B lymphocytes originated in 1960s and 1970s era through experiments conducted in animal models, clinical evaluation of patients having immune system diseases, and the nascent technology of cell surface molecule characterization. In fact, the origin of B. And T. lymphocytes took place simultaneously. The differentiation of the haematopoietic stem cells gives birth to the formation of common lymphoid progenitors, which actually generate B. lymphocytes. They are generated and developed in yolk sac, fetal liver, and the adult liver present in the body (Austyn & Wood, 1994). B lymphocytes are present in areas that come in close contact with foreign substances. They act as defensive mechanism against invading microorganisms, viruses and parasites and play a vital role in humoral immune response. Since these cells originate in the Bone marrow, they are referred as B-cells. B lymphocytes are further differentiated into antibody forming cells which are called plasma cells. These produce glycoproteins which are called immunoglobulins (antibodies).

B-cell development occurs in three main phases. Mature immunocompetent B cells are formed during the initial, antigen independent phase, which can easily bind to a unique antigen. B cells further form immunocompetent naive mature follicular B cells in the bone marrow and spleen during the antigen independent maturation phase. Co-stimulation and antigen binding activates B cells leading towards B cell development in the antigen dependent phase. During this stage, B lymphocytes differentiate into either memory B cells or terminal, antibody-secreting plasma cells. This also involves proliferation of activated B cell, maturation of antibody affinity, and antibody class switching within the secondary lymphoid tissues (Leiber & Tedder, 2008).

A typical B cell antibody molecule is Y shaped, where tips of the Y have identical antigen binding sites and the tail of Y caters the cell-surface receptors. The molecule consists of four polypeptide chains, two identical light chains and two identical heavy chains. Each light and heavy chain is composed of a number of Ig domains -- ? sheet structures containing about 110 amino acids. Heavy chains determine the biological properties of class of antibody which determines what proteins will bind with the antibody.

The Biology of T. Lymphocytes

Thymocytes, T-Cells or Thymus cells are white blood cells which aid in fighting against diseases and infections. They have the distinguishing feature of having a special receptor on their cell surface called T-cell receptors which cater the identification of specific antigens that enter the body. These cells are produced in the bone marrow but mature in the thymus gland. There are several types of T-cells, each one having its own function. Some of them include cytotoxic T-cells, Helper T-cells, memory T-cells, regulatory T-cells, natural killer (NK) T-cells, and Gamma/delta (Y?) T-cells (Austyn & Wood, 1994).

When T cells are present in the bone marrow, they do not posses receptors on their surfaces yet because they do not express CD4 or CD8 glycoproteins (carbohydrate and protein molecules located on the surface of T-cells). Therefore, they are considered double-negative cells (Cd4- Cd8-). T cells become double positive when they become mature. Then MHC molecules help T-cells detect host cells that have been invaded by infectious organisms and then bind to the MCH peptide. T-cells which bind successfully with MCH peptide survive only, while the rest die out. Activated T-cells become helper T-cells, also called CD4 T-cells or effector cells. These cells divide rapidly and secrete proteins called cytokines, which trigger immune cells to engulf the antigen (Beltman et. al, 2007). Mature T-cells are presented with a self-antigen which combines with MHC molecules on antigen-presenting cells (APCs) like B-cells, dendritic cells, and macrophages. The activation of T cells require two signals, first identifies the foreign substance while the other occurs after binding with the invader. T cells reacting too strongly with antigen receive a death signal from the Antigen-Presenting Cells (APC).

Reactions between T. And B cells and other Cell Types

Intercellular communication among various cell types plays an important role in the operation of the immune system. This frequently occurs in the form of physical cell-cell interactions. T cells combine with B cells and complements to combat against infections and diseases. Both B. And T cells interact with antigens in a different way. B cells react with antigens to form antibodies while T cells produce cytokines with antigens. T Helper cells are essential in making some antibody respond. These cells help B cells produce antibody and help other T cells to kill target cells. Helper T cells also provide proteins for the activation of B cells. T cells interact with epithelial cells in the thymus which play an important role in thymus seeding with precursor cells.

Cytokines serve as local mediators, which are secreted by Effector helper T cells, which also contain several costimulatory proteins on their surface. These cytokines and membrane-bound costimulatory proteins influence the behavior of the various cell types they help. Dendritic cells provide foreign peptides to T cells, and the interaction between the two takes 3 minutes approximately. During this reaction T cells crawl around the surface of dendritic cells, searching for the required peptide, which tend to be major histocompatibility complexes (Beltman et. al, 2007).

Importance of Cytokines and their Receptors in Immune System Development and Responses